Hissing noise from front air vents every ~30 seconds with A/C in auto mode
Its cold out right now, so when I start my car, I have the AC set to auto, and when the car warms up and the cabin is approaching the requested temperature, it seems that the AC starts doing something to regulate the temperature, and about every 30 seconds, I will hear a hissing noise coming out of the front air vents. It lasts for about 20-30 seconds, then it is quiet for 20-30 seconds, and this process repeats in a cycle. If i shut off the front air vents (disable the air flow) or I turn the heat nob on the front vents from heating to cooling, the hissing noise goes away. I think when the hissing occurs, I hear something move mechanically behind the dash (like a vent is opening or closing).
I'm not sure where to start with this.